function [x_next, y_next] = f4(x, y, a, b, c, d)
% MNFM4 映射
%   x(k+1) = a*sin(x(k)) + b*sin(c*cos(y(k))*x(k)) + d
%   y(k+1) = y(k) + x(k)

    x_next = a*sin(x) ...
        + b*sin(c*cos(y)*x) ...
        + d;

    y_next = y + x;
end
